Competitive Growth of Degenerate Pattern and Dendrites During Directional Solidification of a Bicrystal Metallic Alloy
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UCTION
DENDRITIC morphologies are found extensively in alloys during solidification processes from casting to welding. Competitive growth between different dendritic structures is a ubiquitous phenomenon that exists during microstructural evolution processes. The outcome of the competition has a significant influence on the final solidification microstructures and plays a crucial role in determining the performance of cast products.[1–5] Currently, the generally accepted model for competitive growth was proposed by Walton and Chalmers[6] and schematically summarized by Rappaz and Gandin.[7] The Walton–Charmers model is based on an undercooling difference during growth of the favorably oriented (FO) dendrites and unfavorably oriented (UO) YUMIN WANG, SHUANGMING LI, ZHENPENG LIU, BIN YANG, and HONG ZHONG are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Shaanxi, Xi’an 710072, P.R. China. Contact e-mail: [email protected]. HUI XING is with the MOE Key Laboratory of Material Physics and Chemistry under Extraordinary, Northwestern Polytechnical University, Xi’an 710129, P.R. China Manuscript submitted April 23, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
dendrites. A lower undercooling ensures that the FO dendrite wins the competition and is not overgrown by UO dendrites during grain growth. It should be noted that the analyses above for competitive growth are exclusively built in the dendritic regime, namely, for FO and UO dendrites. In cubic crystals, well-developed FO and UO dendrites both grow along the direction and the former grow with a small misorientation with respect to the temperature gradient and the latter with a large misorientation. When this misorientation approaches the maximum deviation, i.e., 45 deg, the transition to another type of complex morphology, named a degenerate pattern (also sometimes referred to as ‘‘seaweed-like’’), occurs.[8,9] In such cases, competition during growth occurs between the dendritic and degenerate patterns. In contrast to regular dendrites, the defining characteristic of a degenerate pattern is an unsteady and strongly disordered growth with continuous splitting of the tips, consisting of competing right and left tilted fingers.[10] The foremost tip radius and tip velocity have shown an oscillatory evolution.[11] The degenerate pattern has no stable growth trunk, unlike dendrites, which typically grow along principal crystallographic axes. Hence, it is straightforward to question whether the
classical Walton–Charmers competitive growth model can be applicable for the degenerate pattern because the model has been challenged by experimental and numerical studies.[12–20] It has been observed during grain growth competition that through a mechanism of unusual overgrowth, FO dendrites may be eliminated by their UO neighbors. These unusual phenomena have indicated that the difference in dendrite tip undercooling is not always suitable for predicting the outcome of competitive grain growth. In competitive growth between the degenerat
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